I would regard GrapheneOS in the same way an Linux operating system is refered as a distribution "distro".
You got AOSP and then you got a lot of different distributions of it such as Android, GrapheneOS, CalyxOS, LineageOS and the OEM variations of Android.
The same way the operating system that is free of distributions FreeBSD has several distributions, GhostBSD, NomadBSD.
I think the term distro is very much applicable in this case.